Dishonored Death of the Outsider - The Face of the Abbey Trophy / Achievement Guide - Attend the meeting as Brother Cardoza [Bronze / 30G]

Mission 4
In mission 4, on the top floor of the Royal Conservatory (main objective brings you here), look for a cultist examining a dead body. He is in a side room and as soon as you reach the upper floor a female cultist will tell him that sister Rosewyn is waiting for him.
Sneak up from behind and use the Semblance skill on him. Then walk over to Lena Rosewyn. She has gathered with her followers on the rooftop (she's marked as the main objective). Talk to her and the trophy / achievement will unlock.